This invention relates to a method for the growing of single crystals of metal orthophosphates of the crystallographic point group 32, especially GaPO4 or AlPO4, from a liquid culture medium using seed crystals with surfaces especially selected for the crystals to be grown.
Such crystals are piezoelectric and have a single optical axis, i.e., they have a preferred crystallographic direction, which is designated z or c or as xe2x80x9coptical axisxe2x80x9d. The crystallographic axes X, Y, Z, and the growth surfaces R, r, m, and z of such crystals are shown in FIG. 1. In a position normal to the wall surfaces m there are three symmetry-equivalent Y-axes (only one shown) and normal to the plane Y and Z there are three X-axes (only one shown). The X-axes are polar, i.e., the + and xe2x88x92 directions have different physical properties, such as etching characteristics and velocity of growth. The GaPO4 monocrystal (gallium orthophosphate) has better physical properties than quartz. Particularly to be noted are its piezo effect which is twice as large as in quartz, a higher coupling constant (of interest for surface acoustic wave (SAW) elements) and the absence of an xcex1-xcex2 phase transition at 573xc2x0 C., which permits the material to be used up to 900xc2x0 C.
For the manufacture of large series of sensors or resonators based on a metal orthophosphate, the use of already existing technology facilities would be desirable. Most lithographic processing plants (for instance for the application of SAW layers) are designed for semiconductor materials, where wafers of 3xe2x80x3 diameter or larger are processed. To make good use of the advantages of the new crystals both, from an economic and technological point of view, large undisturbed crystal regions will thus be required.
The absence of natural seeds has given rise to various growth processes in order to obtain single crystals of as large a size as possible. The problem is that the y faces and m faces and the major rhombohedral faces (R faces) will limit growth (see FIG. 1), which means that only seeds of suitably large Y-dimensions will yield satisfactory results. In the direction of the crystallographic X-axes, however, growth is relatively fast.
Terminology for the directions and faces in metal orthophosphates is summed up in tables 1 and 2. Directions are given in the form of surface normals in brackets (the face being specified by the Bravais-Miller index).
From the growth criteria described above it follows that the optimum choice would be a straight seed 1 whose dimensions should be as large as possible in Y-direction, from where the X wing could develop (see FIG. 2, which also shows the symmetry-equivalent axes Yxe2x80x2, Yxe2x80x3 and Xxe2x80x2, Xxe2x80x3). With artificial metal orthophosphates only relatively small dimensions are obtainable when spontaneous crystals are used, even in the case of multiple growth cycles, because the growth rate in Y-direction is extremely low.
The hydrothermal growth process disclosed in AT-B 398 255 will permit use of the longer Y-rods of quartz as seed crystals for producing metal orthophosphates (epitaxy on quartz) . In this way considerable dimensions may be achieved in the X- and Y-directions. During the hydrothermal growth process of the epitaxial method problems will occur in the form of growth disturbances. Next to Brazil twinning, the main disturbance will concern the seed quartz itself, which will be a foreign body inside the grown crystal. This situation is shown in FIG. 3, where individual crystal regions are marked out as seen in the direction of the Y-axis. Inclusions, dislocations and cracks due to mechanical stresses are found in the xe2x80x9cdirect z regionxe2x80x9d 4. On the other hand, the excellent crystal quality of the xe2x80x9cindirect z regionxe2x80x9d 5 has come as a surprise. The xe2x80x9cdirect x regionxe2x80x9d 6 is marked by disturbances in the form of inclusions of the nutrient solution. Depending on conditions of growth, it will be possible nevertheless to use partial regions thereof for applications. The border region between disturbed and undisturbed areas of direct X-growth is formed by the so-called 123 surfaces. These faces are denoted by surface indices {1,2,{overscore (3)},3}. Starting from this region crystals may be grown without the risk of a disturbed direct x region.
Other known methods have been employed to obtain a larger Y dimension by placing several seed crystals one behind the other in a straight line, as discussed in EP-A 0 123 809, for instance, according to which the faces (z faces) selected for crystal growth are arranged in a plane, the individual wafers having a hexagonal cross-section. EP-B1 0 515 288 presents a method of hydrothermal growth using wafer-shaped seed crystals consisting of crystallization nuclei, which are arranged on a planar base element.
Although it has been attempted to use slowly growing surfaces as seed material, this has only served to confirm the slow growth rates. As a consequence the view has been held until now that the use of straight Y-rods will offer the fast growing x faces maximum space for growing and thus help obtain maximum yield.
It is an object of this invention to propose a method for growing large, high-quality crystals of metal orthophosphates, while avoiding growth disturbances as far as possible.
In accordance with the invention this object is achieved by using a seed crystal with at least two rod- or wafer-shaped legs forming an angle with each other and defining a main growth region, which are positioned eccentrically in the single crystal grown. It has been found unexpectedly that conventionally slow-growth surfaces will grow much faster if two contiguous crystallographic faces selected for crystal growing form an angle of  less than 180xc2x0 with each other.
The new process will permit the growing of crystals that are just as large as the ones obtained by using conventional straight seeds, whilst the seed crystal is moved to the edge of the main growth region, thus permitting use of the entire growth zone (e.g., indirect z region). The useful surface area, for example in the X-Y plane, may be doubled in this way, permitting wafers of 3xe2x80x3 diameter to be obtained from metal orthophosphate crystals. This is made possible by a new configuration and alignment of the seed crystals.
The invention provides that the seed crystal be obtained from a single crystal as a monolith forming at least two legs, or that it consist of individual rod- or wafer-shaped single crystal elements whose crystallographic axes are aligned so as to be parallel to each other. Best results are obtained by aligning the axes as accurately as possible, although deviations of 2xc2x0 to 3xc2x0 will be tolerated.
Different seed geometries can be obtained by cuts from a single crystal piece, or by fitting together (xe2x80x9csplicingxe2x80x9d) individual crystal elements. If a splicing technique is used, a most accurate alignment of the legs of the seed crystal is essential, which must be maintained while the seed cystals are beginning to grow.
Best results will be obtained if the seed crystals can be taken from a single piece of crystal, i.e., monolithically. In that instance any problems concerning mechanical stability or configuration will be avoided and the relative alignment of the legs of the seed crystals is provided automatically. Minor errors of absolute orientation (i.e., small rotations in the angle of the seed crystals) to the extent of a few degrees have virtually no influence on the quality of growth.
It is further provided by the invention that the seed crystal consist of crystal material that is a homeotype of the single crystal to be grown. GaPO4, AlPO4, FePO4, GaAsO4, AlAsO4, SiO2, or GeO2 are the materials preferred in this context. With the use of an epitaxial process a thin layer of the desired crystal material is applied. This process must be performed in two steps, however. The advantage of the epitaxial method is that quartz seeds can be used which are considerably larger and less costly than seed crystals based on metal orthophosphates.
In the instance of a seed crystal with rod-shaped legs it is preferable for the main axes of the legs to define a plane normal to the crystallographic Z-axis.